The present invention relates to fiber Raman amplifiers.
The subject of Raman amplification is well known in the literature. Stimulated Raman amplification is a nonlinear optical process in which an intense pump wave is injected into a medium such as an optical fiber that is carrying one or more optical signals. In fused silica fibers, if the pump wave is of a frequency approximately 13 THz greater than the signal waves, the pump will amplify the signal(s) via stimulated Raman scattering. For example, a pump with a wavelength of 1450 nm will amplify an optical signal at a wavelength of approximately 1550 nm.
Various designs of fiber Raman amplifiers have been used over the past few years, where the gain characteristics of the amplifier have been tailored by changing the pump power, fiber length and fiber composition. In some arrangements, the pump and signals may be xe2x80x9cco-propagatingxe2x80x9d through the fiber. In most cases, however, the pump and signal are xe2x80x9ccounter-propagatingxe2x80x9d, since this arrangement will allow for the signal to be amplified while minimizing pump-signal cross talk. Moreover, counter-propagating arrangements permit the use of optical circulators, which provide combined multiplexing and isolator functions.
Most of the single pump, single signal amplifiers constructed to date have been optimized for use when the difference between the pump and the signal frequency is tuned near the peak of the Raman gain coefficient (i.e., the 13 THz value mentioned above for fused silica). However, since the Raman gain coefficient in silica- and germanium-based optical fibers begins at the pump frequency, signal amplification is also possible at much smaller separations. More recently, multiple pump wavelengths have been used to further extend the amplification bandwidth. Gain flattening filters have also been used with either single or multiple pumps to provide a relatively flat amplification window for multiple signals.
To date, however, conventional Raman amplifiers display a significant drawback in that any xe2x80x9coptical noisexe2x80x9d present in the system will also be amplified. The origins of the optical noise may be side modes of the signal or pump(s), or the optical noise created by spontaneous Raman scattering of the pump(s). The amount of noise amplification depends on the frequency shift of the optical noise away from the pump(s) and on the pump power(s). In a strongly pumped amplifier, the noise amplification can be large enough to cause pump depletion. Furthermore, the optical noise at frequency shifts higher than the signal can absorb some of the energy contained in the signal and lead to signal depletion.
Thus, a need remains in the art for overcoming the problems associated with optical noise in fiber Raman amplifiers.
The need remaining in the prior art is addressed by the present invention, which relates to fiber Raman amplifiers and, more particularly, to the inclusion of one or more high pass optical filters in the amplifier to enhance its gain and improve the pump conversion efficiency.
In accordance with the present invention, a high pass optical filter is included in the amplifier design, where the filter exhibits a cut-off frequency immediately below the signal frequency, with low loss at this frequency and high loss at the unwanted optical noise frequencies. In arrangements utilizing multiple input signals, the high pass filter is configured to exhibit a cut-off frequency immediately below the lowest signal frequency. The resultant high loss at Raman noise frequencies will lead to reduced interactions of the optical noise with the pump(s) and signal(s). Therefore, the signal gain of the amplifier will increase and the pump conversion efficiency will improve.
In one embodiment, the high pass filter may comprise a discrete element, such as a fused silica coupler, a dielectric stack, or long period Bragg gratings. Multiple filters may be utilized in order to reduce noise interaction along the length of the fiber.
In an alternative embodiment, the high pass filter may be of the xe2x80x9cdistributedxe2x80x9d type, achieved by inserting absorbing ions into the core of the transmission fiber, or placing an absorbing layer around the core of the fiber.
High pass optical filtering in accordance with the present invention may also be used in xe2x80x9cremotexe2x80x9d pumping applications, where the pump input is physically displaced from the amplifying medium. The high pass filter is designed to exhibit a cut-off frequency immediately below the pump frequency, with low loss at this frequency and high loss at the unwanted optical noise frequencies. In arrangements utilizing multiple input pumps, the high pass filter is configured to exhibit a cut-off frequency immediately below the lowest pump frequency. The resultant high loss at Raman noise frequencies will lead to reduced interactions of the optical noise with the pump(s), thus permitting greater pump power to reach the amplifying medium. Thus, the signal gain of the amplifier will increase and the pump conversion efficiency will improve.
High pass optical filtering is also useful in eliminating noise (and thus increasing gain and improving pump conversion efficiency) in so-called xe2x80x9csecond orderxe2x80x9d Raman amplifiers, where a first pump is used to amplify a second pump and the second pump is then used to amplify one or more information signals.
Various and other embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.